Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal, acoustic etc), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
U.S. Pat. No. 7,864,992 discloses a fingerprint sensing system in which a driving signal is injected into the finger by pulsing a conductive structure arranged in the vicinity of the sensor array and measuring the resulting change of the charge carried by the sensing structures in the sensor array.
Although the fingerprint sensing system according to U.S. Pat. No. 7,864,992 provides for an excellent combination of fingerprint image quality and sensor protection, it would, in some applications be desirable to be able to acquire a high-quality fingerprint image without having to change the potential of the finger. In particular, there appears to be room for improvement for “difficult” fingers, such as dry fingers.
US 2015/0015537 discloses a fingerprint sensing system in which the reference potential of the sensor array is allowed to swing in relation to a device reference potential. Since the finger potential of a finger on the sensor array can be assumed to be constant or vary slowly in relation to the device reference potential, the swing of the reference potential of the sensor array can replace the swing of the finger potential used by the fingerprint sensing system according to U.S. Pat. No. 7,864,992. Since the approach according to US 2015/0015537 is practically independent of the conductivity of the finger, better results may be obtained for the above-mentioned “difficult” fingers.
U.S. Pat. No. 9,152,841 discloses another fingerprint sensing system in which the local reference potential of single sensing elements or groups of sensing elements is controlled to swing in relation to a device reference potential. According to U.S. Pat. No. 9,152,841, each sensing element comprises a sense transistor formed in a well, and the well is controlled to swing together with the sensing structure of the sensing element, to reduce the influence of parasitic capacitances.
It would be desirable to provide for even further improved fingerprint sensing, in particular through a thicker dielectric structure.